Images of various extrusive igneous rocks

Virtual Collection: Extrusive Igneous Rocks

Virtual Rock Collection

Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks

Image above: Examples of different types of extrusive igneous rocks. Sources of individual rock images are derived from 3D models on Sketchfab with Creative Commons licensing; see same models below for creator credits and licensing details.

Overview

Extrusive (or, volcanic) igneous rocks are formed on the Earth’s surface as the result of volcanic activity. They tend to have small crystal sizes because they cool very quickly. These small crystals are referred to as aphanitic because they often cannot be seen with the naked eye. That said, larger, visible crystals called phenocrysts may be present within the surrounding groundmass (or, matrix) of aphanitic crystals. Extrusive igenous rocks with phenocrysts in an aphanitic matrix are called porphyry; they form when magma has partially cooled below the surface (forming the phenocrysts) and is then erupted and rapidly cooled (forming the aphenitic matrix).

The colors of extrusive igneous rocks depend upon the minerals present. Felsic extrusive igneous rocks (e.g., rhyolite) are usually light in color (though obsidian is a notable exception; see below) because they are rich in minerals like feldspar that contain a lot of Al, Na, or K. Mafic extrusive igneous rocks (e.g., basalt) are darker in color, again reflecting the compositions of the minerals present, which tend to be rich in Mg and Fe. There are, of course, extrusive igenous rocks with compositions that are somewhere between felsic and mafic (e.g., dacite and andesite).

Crystalline Extrusive Igneous Rocks

These igneous rocks have textures composed of interlocking crystals, usually of varying mineral composition. Rhyolite, dacite, andesite, and basalt are crystalline extrusive igneous rocks form a continuum of composition from felsic to mafic and are presented in this order below.

Rhyolite

Rhyolite is the extrusive equivalent of granite. It is felsic in composition and contains large amounts of potassium feldspar and quartz, commonly giving it a pink color.


Sample of rhyolite from Castle Rock, Colorado. Longest dimension of sample is approximately 11 cm. Specimen is from the teaching collections of the Paleontological Research Institution, Ithaca, NY (Sketchfab; Creative Commons Attribution-ShareAlike 4.0 International license).

Dacite

Dacite is the extrusive equivalent of granodiorite. Quartz and plagioclase feldspar are abundant minerals, often giving dacite a grayish color.


Sample of dacite. Model by EduRock - Educational Virtual Rock Collection (Sketchfab; Creative Commons Attribution 4.0 International license).

Andesite

Andesite is the extrusive equivalent of diorite. Plagioclase feldspar is abundant, but quartz is nearly absent; mafic minerals are present, giving this type of rock a dark color. 


Original caption: "This andesite comes from Mineral County, Nevada. It contains mostly fine-grained plagioclase and black hornblende phenocrysts. The darker matrix contains hornblende, and minor K-feldspar, augite and quartz – but they cannot be seen in hand sample. This specimen is 8 cm across." Model by "rocksandminerals" (Sketchfab; Creative Commons Attribution 4.0 International license).

Basalt

Basalt is the extrusive equivalent of gabbro. It is dark in color and dominated by mafic minerals such as pyroxene. Most magmas that erupt from volcanos form basalt.


Sample of basalt. Model by EduRock - Educational Virtual Rock Collection (Sketchfab; Creative Commons Attribution 4.0 International license).

Sample of basalt from Hawaii with a smooth "pahoehoe" form. Longest dimension of specimen is approximately 8 cm (Sketchfab; Creative Commons Attribution-ShareAlike 4.0 International license).


Sample of basalt from Hawaii with a sharp "A'a" form. Longest dimension of specimen is approximately 15 cm (Sketchfab; Creative Commons Attribution-ShareAlike 4.0 International license).

Fragmental Extrusive Igneous Rocks

Fragmental extrusive igenous rocks are formed from fragments that are exploded out of the Earth, settle, and then become welded together as they quickly cool. Tuff is an example.

Tuff

A tuff is an extrusive igneous rock that has formed from lithified volcanic ash. Tuffs often appear as layered rocks at the outcrop scale.


Sample of the Brisbane Tuff that includes an imprint of a plant stem. Model by Nate Siddle and The University of Queensland School of Earth and Environmental Sciences (Sketchfab).

Original caption: "This outcrop is the lower section of the cliff at Swansea Heads near Newcastle, Australia. Most of the section shown is the thick (~10 metres) tuff unit of the Reids Mistake Member of the Newcastle Coal Measures - a coal seam can be seen toward the top of the section. Coal also underlies the tuff, and the preserved remains of dozens of Glossopteris trees are present in the lower parts of the tuff on the rock platform." Model by Earth Sciences, University of Newcastle (Sketchfab).

Glassy Extrusive Igneous Rocks

Glassy extrusive igenous rocks contain a large amount of silica (Si) and form when lava cools very quickly, preventing the formation of individual mineral grains. Examples include obsidian, pumice, and scoria.

Obsidian

Obsidian is volcanic glass composed almost entirely of silica, making it felsic, despite its color. Scallop-shaped conchoidal fractures are common on obsidian samples.


Sample of obsidian from Millard County, Utah. Specimen is from the teaching collections of the Paleontological Research Institution, Ithaca, New York. Longest dimension of specimen is approximately 12 cm (Sketchfab; Creative Commons Attribution-ShareAlike 4.0 International license).

Pumice

Pumice is a glassy extrusive igneous rock that has many small holes called vesicles, the result of air bubbles that were present in erupted lava before it cooled. Some samples are so porous that they float when placed in water.


Original caption: "This sample of pumice contains clear colorless glass shards, but they are very difficult to see. There is also a minor amount of darker glass. The specimen 9 cm x 6 cm and comes from Millard County, Utah." Model by "rocksandminerals" (Sketchfab; Creative Commons Attribution 4.0 International license).

Scoria

Scoria is similar to pumice, but has larger vesicles (holes). It is often reddish in color.


Sample of scoria from Rocoş, Romania. Model by the Museum of Mineralogy and Petrography, UAIC (Sketchfab).


Sample of scoria from Klamath Falls, Oregon. Specimen is from the teaching collections of the Paleontological Research Institution, Ithaca, New York. Longest dimension of specimen is approximately 8.5 cm (Sketchfab; Creative Commons Attribution-ShareAlike 4.0 International license).